Liquid crystal display device

ABSTRACT

A positive uniaxial film  14  with a retardation of Rp [nm] in an in-plane direction is disposed between a vertical alignment mode liquid crystal cell  11  and a polarizing plate  12 , and a negative uniaxial film  15  with a retardation of Rn [nm] in a thickness direction is disposed between the positive uniaxial film  14  and the polarizing plate  12 . Further, when a parameter α 1 [ nm] in relation to Rp is 35+(Rlc/80−4) 2 ×3.5+(360−Rlc)×Rtac/850; and a parameter β 1 [ nm] in relation to Rn is Rlc−1.9×Rtac, where Rtac [nm] is a retardation in a thickness direction of the respective base films of polarizing plates  12  and  13 , the retardations Rp and Rn are set to fall within ranges of 80% to 120% of the parameter α 1  and 60% to 90% of the parameter β 1 , respectively.

This application is a continuation of U.S. patent application Ser. No.10/467,765 filed Aug. 12, 2003 (U.S. Pat. No. 6,885,421, issued Apr. 26,2005) which is the US national phase of international applicationPCT/JP03/02521 filed 4 Mar. 2003, which designated the US and claimspriority from JP application number 2003-064481 filed Mar. 8, 2002, allof which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a vertical alignment mode liquidcrystal display device.

BACKGROUND ART

Conventionally, a liquid crystal display device has been widely used forthe screen of a word processor and a computer. In recent years, theliquid crystal display device has spread rapidly as a television screen.Many of these liquid crystal display devices adopt TN (Twisted Nematic)mode. However, the TN mode liquid crystal display device has theproblems of tendencies to degrade a contrast and to reverse a gradationproperty when viewed from an oblique direction.

For this reason, in recent years, in light of the improvement in viewingangle property when viewed from an oblique direction, a VA (VerticalAlignment) mode liquid crystal display device has been attractingattention. A liquid crystal cell of the VA mode liquid crystal displaydevice is arranged in combination of a nematic liquid crystal having anegative dielectric anisotropy with a vertical alignment film.

Further, for example, Japanese Patent No. 2947350 (published on Sep. 13,1999) and Japanese Laid-Open Patent Publication No. 2000-39610(published on Feb. 8, 2000), as shown in FIGS. 16 and 17, disclose aliquid crystal display device 101 in which a biaxial film 116 isprovided between a liquid crystal cell 111 and a polarizing plate 112,and a liquid crystal display device 101 a in which a positive uniaxialfilm 114 and a negative uniaxial film 115 are provided between theliquid crystal cell 111 and the polarizing plate 112 and between thepositive uniaxial film 114 and the polarizing plate 112, respectively,in order to optically compensate for the optical anisotropy of theliquid crystal cell 111 when black image is displayed.

With the above arrangement, even though the liquid crystal cell 111brings a phase difference depending on a polar angle to transmittinglight when the liquid crystal cell 111 in which liquid crystal moleculesare oriented vertically is viewed from an oblique direction, the phasedifference can be compensated by properly setting the respectiveretardations of the film 116 (films 114 and 115). Therefore, a blackdisplay can be performed substantially as in the case when the liquidcrystal cell 111 is viewed from a front direction, that is, as in thecase where the liquid crystal molecules maintain the polarized state ofthe transmitting light. As a result of this, it is possible to preventlight leakage, thus enhancing the contrast and suppressing theoccurrence of coloring and tone degradation when viewed from an obliquedirection.

However, nowadays, under the situation where a liquid crystal displaydevice with a wider viewing angle and higher display quality isexpected, there is a demand for the improvement in coloring and tonedegradation caused when viewed from the oblique direction. The liquidcrystal display device using the films 116 (114 and 115) with theretardation described in Japanese Patent No. 2947350 and JapaneseLaid-Open Patent Publication No. 2000-39610, however, is not alwayssatisfactory and still leaves room for improvement.

In view of the above problems, the present invention in a verticalalignment mode liquid crystal display device is attained as a result ofinvestigating the effects of base films of polarizing plates on suitableretardations of films for the suppression of coloring and tonedegradation when viewed from an oblique direction. It is therefore anobject of the present invention is to surely provide a liquid crystaldisplay device in which the coloring and tone degradation can besuppressed within allowable limits in practical use, while maintaining acontrast at a sufficiently high value in practical use when viewed froman oblique direction.

DISCLOSURE OF INVENTION

In order to achieve the above object, a liquid crystal display deviceaccording to the present invention, includes:

a liquid crystal cell having a pair of substrates and liquid crystalinterposed therebetween, wherein liquid crystal molecules of the liquidcrystal are oriented substantially vertically to respective surfaces ofthe pair of substrates;

a pair of polarizing plates disposed so as to sandwich the liquidcrystal cell therebetween, respective absorption axes of the pair ofpolarizing plates being orthogonal to each other;

a first phase difference film, disposed between one of the pair ofpolarizing plates and the liquid crystal cell, the first phasedifference film having a positive uniaxial anisotropy; and

a second phase difference film, disposed between the one of the pair ofpolarizing plates and the first phase difference film, the second phasedifference film having a negative uniaxial anisotropy,

wherein each of the pair of polarizing plates has a base film with anoptical axis substantially vertical to the pair of substrates, the basefilm having a negative uniaxial anisotropy, the first phase differencefilm has a retardation axis crossing at right angle the absorption axisof the one of the pair of polarizing plates on the same side when seenfrom the liquid crystal, and the second phase difference film has anoptical axis substantially vertical to the pair of substrates, takingthe following means.

More specifically, when a parameter α [nm] in relation to Rp is:α=35+(Rlc/80−4)²×3.5+(360−Rlc)×Rtac/850; and

a parameter β [nm] in relation to Rn is:β=Rlc−1.9×Rtac,

where Rp [nm] is a retardation in an in-plane direction of the firstphase difference film, Rn [nm] is a retardation in a thickness directionof the second phase difference film, Rtac [nm] is a retardation in athickness direction of the base films, and Rlc [nm] is a retardation ina thickness direction of the liquid crystal,

the retardation Rp is set to fall within the range from not less than80% to not more than 120% of the parameter α, and the retardation Rn isset to fall within the range from not less than 60% to not more than 90%of the parameter β.

In the above-arranged liquid crystal display device, liquid crystalmolecules oriented substantially vertically to the substrates, althoughnot bringing a phase difference to light incident from the normaldirection to the substrate, bring a phase difference depending on apolar angle (tilt angle to the normal direction) to obliquely incidentlight. Therefore, the liquid crystal display device cannot completelyabsorb the light supposed to be absorbed by the polarizing plate on theside from where the light emits, without the first and second phasedifference films. This results in the occurrence of light leakage, thusdegrading a contrast and causing the coloring and tone degradation.

In order to solve the problem, since the above arrangement is providedwith first and second phase difference films, the phase difference thatthe liquid crystal has brought depending on the polar angle can becompensated by the first and second phase difference films. As a resultof this, it is possible to prevent light leakage when viewed from anoblique direction, enhancing the contrast and preventing the occurrenceof coloring and tone degradation.

Incidentally, when the respective retardations of the first and secondphase difference films are determined, it cannot be always said thatjust subtracting the retardation in the thickness direction of the basefilms from each of the respective retardations in the thicknessdirection of the first and second phase difference films, which is anoptimum retardation when base films are absent, is sufficient, becausethe coloring and tone degradation caused when viewed from an obliquedirection are required to be suppressed much further.

The inventors of the present application, as a result of extensiveresearch to further suppress the coloring and tone degradation, whilemaintaining a contrast at a sufficiently high value in practical usewhen a vertical alignment mode liquid crystal display device is viewedfrom an oblique direction, have found that the retardation in thethickness direction of the base films does not always function asequally as each of the retardation in the thickness direction of thefirst phase difference film and the retardation in the thicknessdirection of the second phase difference film. Specifically, theinventors have found to complete the present invention that: when theretardation in the in-plane direction of the first phase difference filmwith a positive uniaxial anisotropy is set so that the contrast becomesthe maximum, the dependency of the retardation Rp on the retardation inthe thickness direction of the base films, reverses depending on whetherthe retardation of the liquid crystal is over 360 [nm], and it ispossible to effectively suppress the coloring and tone degradation bysetting the retardations to fall within a predetermined range withreference to such retardations that the contrast becomes the maximum.

In the liquid crystal display device of the present invention, theretardations Rp and Rn are set according to the retardation Rtac in thethickness direction of the base films and the retardation Rlc in thethickness direction of the liquid crystal, and the retardations Rp andRn are set to fall within the range where the coloring and tonedegradation can be tolerated, while maintaining a contrast at asufficiently high value in practical use when viewed from an obliquedirection. With this arrangement, unlike the arrangement in which theretardation in the thickness direction of the base films is treatedequally to the retardation in the thickness direction of the first phasedifference film and the retardation in the thickness direction of thesecond phase difference film, it is possible to surely obtain a liquidcrystal display device which can maintain a contrast at a sufficientlyhigh value in practical use when viewed from the oblique direction andlimit the coloring and tone degradation within allowable limits.

In the case where the improvement in productivity is especiallyrequired, in addition to the above arrangement, it is desirable that theretardation Rlc in the thickness direction of the liquid crystal is setto fall in the range from 324 [nm] to 396 [nm], and the retardation Rpin the in-plane direction of the first phase difference film is set tofall within a range from 30.7 [nm] to 41.7 [nm].

If the retardation Rlc is set to fall within the foregoing range, thedependency of the retardation Rp on the retardation in the thicknessdirection of the base films is small. Therefore, even in the case wherethe variations of the base films caused in the manufacturing processvaries the retardation in the thickness direction of the base films, theretardation Rp can be set to fall within the range from 80% to 120% ofthe parameter α by setting the retardations Rlc and Rp to be in theforegoing ranges. As a result of this, even in the case where theretardation in the thickness direction of the base films varies, it ispossible to use the same first phase difference film, thus improving theproductivity.

Further, in the case where the suppression of coloring and tonedegradation is especially required, in addition to the abovearrangement, it is desirable that the retardation Rp is set to fallwithin the range from not less than 90% to not more than 110% of theparameter α, and the retardation Rn is set to fall within the range fromnot less than 65% to not more than 85% of the parameter β. With thisarrangement, it is possible to obtain a liquid crystal display devicewhich can further suppress the coloring and tone degradation when viewedfrom an oblique direction.

Still further, in the case where both the suppression of coloring andtone degradation and the improvement in productivity are especiallyrequired, it is desirable that the retardation Rlc in the thicknessdirection of the liquid crystal is set to fall within the range from 342[nm] to 378 [nm], and the retardation Rp in the in-plane direction ofthe first phase difference film is set to fall within the range from33.3 [nm] to 38.6 [nm].

If the retardations Rlc and Rp are set to fall within the foregoingranges, the retardation Rp can be set to fall within the range from 90%to 110% of the parameter α even in the case where the variations of thebase films caused in the manufacturing process varies the retardation inthe thickness direction of the base films. As a result of this, even inthe case where the retardation in the thickness direction of the basefilms varies, it is possible to use the same first phase differencefilm, thus improving the productivity.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the main arrangement of a liquidcrystal display device according to an embodiment of the presentinvention.

FIG. 2 is a schematic diagram showing a liquid crystal cell provided inthe liquid crystal display device in the state where no voltage isapplied.

FIG. 3 is a schematic diagram showing a liquid crystal cell provided inthe liquid crystal display device in the state where a voltage isapplied.

FIG. 4 is a plan view showing the vicinity of a pixel electrode in anarrangement example of the liquid crystal cell.

FIG. 5 shows preferable ranges for retardation in the in-plane directionof a positive uniaxial film and retardation in the thickness directionof a negative uniaxial film which are provided in the liquid crystaldisplay device, where the retardations are expressed by relative valuesto the respective parameters.

FIG. 6 shows experimental results of optimal values for the retardationswith respect to the combination of a liquid crystal cell with apolarizing plate in an example of the present invention.

FIG. 7 shows a contrast evaluation method in a liquid crystal displaydevice.

FIG. 8 is a schematic diagram showing the main arrangement of a liquidcrystal display device according to another embodiment of the presentinvention.

FIG. 9 shows preferable ranges for retardations in the in-planedirection and thickness direction of a biaxial film which is provided inthe liquid crystal display device, where the retardations are expressedby relative values to the respective parameters.

FIG. 10 is a schematic diagram showing the main arrangement of a liquidcrystal display device which is a modified example of the foregoingliquid crystal display device.

FIG. 11 shows experimental results of optimal values for theretardations with respect to the combination of a liquid crystal cellwith a polarizing plate in an example of the present invention.

FIG. 12 is a perspective view showing a pixel electrode of a liquidcrystal cell in another arrangement example of the foregoing liquidcrystal display devices.

FIG. 13 is a plan view showing the vicinity of a pixel electrode in aliquid crystal cell in still another arrangement example of theforegoing liquid crystal display devices.

FIG. 14 is a perspective view showing a pixel electrode of a liquidcrystal cell in yet another arrangement example of the foregoing liquidcrystal display devices.

FIG. 15 is a perspective view showing a pixel electrode and a counterelectrode of a liquid crystal cell in still another arrangement exampleof the foregoing liquid crystal display devices.

FIG. 16 is a schematic diagram showing the main arrangement of a liquidcrystal display device with a conventional art.

FIG. 17 is a schematic diagram showing the main arrangement of a liquidcrystal display device with another conventional art.

BEST MODE FOR CARRYING OUT THE INVENTION FIRST EMBODIMENT

Referring to FIGS. 1 to 7, the following will explain one embodiment ofthe present invention. Note that, as described in detail later, thepresent invention can be applied to other liquid crystal cell; however,the following will explain a multi-domain vertical alignment liquidcrystal cell as one preferable example of a liquid crystal cell.

A liquid crystal display device 1 according to the present embodiment,as shown in FIG. 1, has a layer structure, including a verticalalignment (VA) mode liquid crystal cell 11, polarizing plates 12 and 13which are respectively disposed so as to sandwich the liquid crystalcell 11 therebetween, a positive uniaxial film (first phase differencefilm) 14 which is disposed between the polarizing plate 12 and theliquid crystal cell 11, and a negative uniaxial film (second phasedifference film) 15 which is disposed between the positive uniaxial film14 and the polarizing plate 12.

The liquid crystal cell 11, as shown in FIG. 2, includes: a TFT (ThinFilm Transistor) substrate 11 a provided with a pixel electrode 21 acorresponding to a pixel (described later); a counter substrate 11 bprovided with a counter electrode 21 b; and a liquid crystal layer 11 c,sandwiched between the substrates 11 a and 11 b, which is a nematicliquid crystal with negative dielectric anisotropy. Note that, theliquid crystal display device 1 according to the present embodiment iscapable of color display, and a color filter including the colors ofpixels is formed on the counter substrate 11 b.

Further, under the TFT substrate 11 a provided with the pixel electrode21 a, a vertical alignment film 22 a is formed on one side surface ofthe liquid crystal layer 11 c. Similarly, under the counter substrate 11b provided with the counter electrode 21 b, a vertical alignment film 22b is formed on the other side surface of the liquid crystal layer 11 c.With this arrangement, in the state where a voltage is not appliedbetween the two electrodes 21 a and 21 b, liquid crystal molecules M inthe liquid crystal layer 11 c, which is disposed between the twosubstrates 11 a and 11 b, are oriented vertically to the surfaces of thesubstrates 11 a and 11 b. On the other hand, when a voltage is appliedbetween the two electrodes 21 a and 21 b, the liquid crystal molecules Min the state of being oriented in the normal direction to the substrates11 a and 11 b (in the state where no voltage is applied) tilt at a tiltangle depending on the applied voltage (see FIG. 3). Note that, the twosubstrates 11 a and 11 b are opposed to each other, so that the normaldirection and in-plane direction with respect to the substrates 11 a and11 b are hereinafter referred to just as normal direction and in-planedirection, except for the case where they are required to bedistinguished.

Here, the liquid crystal cell 11 according to the present embodiment isa multi-domain vertical alignment liquid crystal cell in which eachpixel is divided into a plurality of regions (domains), and iscontrolled so that the domains have mutually different orientationdirections, i.e. azimuths of the liquid crystal molecules M tilted whena voltage is applied (in-plane components at a tilt angle).

More specifically, as shown in FIG. 4, the pixel electrode 21 a hasprotrusions 23 a formed thereon in a stripe pattern, and each of theprotrusions 23 a zigzags appropriately at right angles in the in-planedirection with a mountain shape in cross-section. Similarly, the counterelectrode 21 b has protrusions 23 b formed thereon in a stripe pattern,and each of the protrusions 23 b zigzags appropriately at right anglesin the in-plane direction with a mountain shape in the normal direction.The two protrusions 23 a and 23 b in the in-plane direction are disposedat such a distance that the normal to the slope of the protrusion 23 ais substantially equal to the normal to the slope of the protrusion 23b. Further, the protrusions 23 a and 23 b are formed by the applicationof a photosensitive resin on the pixel electrode 21 a and the counterelectrode 21 b, respectively, and the process of photolithography.

Here, the liquid crystal molecules near the protrusions 23 a areoriented so as to be vertical to the slopes of the protrusions 23. Inaddition, when a voltage is applied, the electric field near theprotrusions 23 a tilts so as to be parallel to the slopes of theprotrusions 23 a. Here, the long axes of the liquid crystal moleculestilt in the vertical direction with respect to the electric field.According to the continuity of liquid crystal, the liquid crystalmolecules away from the slopes of the protrusions 23 a are oriented aswell in the same direction as the direction in which the liquid crystalmolecules near the slopes of the protrusions 23 a tilt. Similarly, whena voltage is applied, the electric field near the protrusions 23 b tiltsso as to be parallel to the slopes of the protrusions 23 b. Here, thelong axes of the liquid crystal molecules tilt in the vertical directionwith respect to the electric field. According to the continuity ofliquid crystal, the liquid crystal molecules away from the slopes of theprotrusions 23 b are oriented as well in the same direction as thedirection in which the liquid crystal molecules near the slopes of theprotrusions 23 b tilt.

Consequently, as to the protrusions 23 a and 23 b, when parts except fora corner part C are referred to as line parts, in the region between aline part L23 a of the protrusion 23 a and a line part L23 b of theprotrusion 23 b, in-plane component as the orientation direction of theliquid crystal molecules when a voltage is applied is equal to that inthe direction from the line part L23 a to the line part L23 b.

Here, in the protrusions 23 a and 23 b, the corner part C bendsapproximately at right angles. Therefore, the orientation directions ofthe liquid crystal molecules are divided into four parts in a pixel, andthis can form domains D1 to D4 of mutually different orientationdirections of the liquid crystal molecules in the pixel.

On the other hand, the polarizing plates 12 and 13 shown in FIG. 1 arerespectively provided with polarizing films 12 a and 13 a and triacetylcellulose (TAC) films 12 b and 13 b as base films for holding thepolarizing films 12 a and 13 a. The two TAC films 12 b and 13 b havenegative optically uniaxial anisotropy, and the respective optical axesare set so as to be substantially equal to the normal direction of theliquid crystal cell 11. The two polarizing plates 12 and 13 are disposedso that an absorption axis AA12 of the polarizing plate 12 is orthogonalto an absorption axis AA13 of the polarizing plate 13. Further, the twopolarizing plates 12 and 13 are disposed so that each of the absorptionaxes AA12 and AA13 forms an angle of 45 degrees with the in-planecomponent in the orientation direction of the liquid crystal moleculesin each of the domains D1 to D4 when a voltage is applied.

Further, the positive uniaxial film 14, which is layered on one surfaceof the liquid crystal cell 11, is an optically anisotropic film havingthe property of nxp>nyp=nzp, where nxp and nyp are refractive indexes inthe in-plane direction of the film, and nzp is a refractive index in thenormal direction. Let a film thickness be dp, retardation Rp in thein-plane direction can be calculated by the following expression (1):Rp=dp·(nxp−nyp)  (1).Further, the positive uniaxial film 14 is disposed so that itsretardation axis SL14 is orthogonal to the absorption axis AA12 of thepolarizing plate 12 at the same side when viewed from the liquid crystalcell 11.

Meanwhile, the negative uniaxial film 15, which is layered on the othersurface of the liquid crystal cell 11, is an optically anisotropic filmhaving the property of nxn=nyn>nzn, where nxn and nyn are refractiveindexes in the in-plane direction of the film, and nzn is a refractiveindex in the normal direction. Let a film thickness be dn, retardationRn in the thickness direction can be calculated by the followingexpression (2):Rn=dn·{(nxn+nyn)/2−nzn}  (2).Further, the negative uniaxial film 15 is disposed so that its opticalaxis is substantially equal to the normal direction of the liquidcrystal cell 11.

With the above-arranged liquid crystal display device 1, while a voltageis applied between the pixel electrode 21 a and the counter electrode 21b, the liquid crystal molecules in the liquid crystal cell 11, as shownin FIG. 3, are obliquely oriented to the normal direction just at theangle depending on the applied voltage. This brings a phase differencedepending on the applied voltage to light passing through the liquidcrystal cell 11.

Here, the absorption axes AA12 and AA13 of the polarizing plates 12 and13 are disposed so as to be orthogonal to each other. As described indetail later, the positive uniaxial film 14 and negative uniaxial film15 are arranged so as to compensate the phase difference that the liquidcrystal cell 11 brings to transmitting light in the case where theliquid crystal molecules in the liquid crystal cell 11 are oriented inthe normal direction, as shown in FIG. 2.

Therefore, light incident to a polarizing plate on the side from wherethe light emits (e.g. the polarizing plate 12) turns ellipticallypolarized light depending on the phase difference that the liquidcrystal cell 11 brings, and a part of the incident light passes throughthe polarizing plate 12. As a result of this, the amount of lightemitted from the polarizing plate 12 can be controlled in accordancewith the applied voltage. This makes it possible to display withgradations.

Further, the liquid crystal cell 11 has domains D1 to D4 formed ofmutually different orientation directions of the liquid crystalmolecules in a pixel. Therefore, even in the case where the liquidcrystal molecules cannot bring phase difference to transmitting lightwhen the liquid crystal cell 11 is viewed from the direction which isparallel to the orientation direction of liquid crystal molecules whichbelong to a certain domain (e.g. the domain D1), the liquid crystalmolecules in the rest of domains (the domains D2 to D4 in this case) canbring phase difference to transmitting light. Thus, the domains canoptically compensate with one another. As a result of this, it ispossible to improve a display quality level of the liquid crystal cell11 and to increase a viewing angle when viewed from an obliquedirection.

On the other hand, while a voltage is not applied between the pixelelectrode 21 a and the counter electrode 21 b, the liquid crystalmolecules in the liquid crystal cell 11, as shown in FIG. 2, are in thestate of being oriented vertically. In this state (when no voltage isapplied), the light incident from the normal direction to the liquidcrystal cell 11, which cannot be brought phase difference by the liquidcrystal molecules, passes through the liquid crystal cell 11,maintaining a polarized state. As a result of this, light incident to apolarizing plate on the side from where the light emits (e.g. thepolarizing plate 12) turns linearly polarized light which issubstantially parallel to the absorption axis AA12 of the polarizingplate 12 and cannot pass through the polarizing plate 12. This allowsthe liquid crystal display device 1 to display black image.

Here, to the light incident from an oblique direction to the liquidcrystal cell 11, brought by liquid crystal molecules is the phasedifference depending on the angle between the incident light and theorientation direction of the liquid crystal molecules, that is, theangle (polar angle) between the incident light and the normal directionto the liquid crystal cell 11. Therefore, without the positive uniaxialfilm 14 and the negative uniaxial film 15, the light incident to thepolarizing plate 12 turns elliptically polarized light depending on thepolar angle, and a part of the polarized light passes through thepolarizing plate 12. This results in the occurrence of light leakageeven in the state where the liquid crystal molecules are orientedvertically to display black image, which could degrade display contrastand cause the coloring and tone degradation.

However, with the arrangement shown in FIG. 1 in which the positiveuniaxial film 14 and the negative uniaxial film 15 are provided, if therespective retardations are properly set, the phase difference broughtdepending on the polar angle by the liquid crystal cell 11 can becancelled. As a result of this, it is possible to prevent light leakage,thus enhancing the contrast and suppressing the occurrence of coloringand tone degradation when viewed from an oblique direction.

Here, in the liquid crystal display device 1 according to the presentembodiment, the respective retardations of the positive uniaxial film 14and the negative uniaxial film 15 are set as described below, in orderto attain a liquid crystal display device which characterizes excellentcolor and gradation, maintaining a sufficiently high contrast inpractical use, as a display quality level when viewed obliquely; morespecifically, in order to attain a liquid crystal display device suchthat a viewer hardly perceives the coloring and tone degradation whenviewing from an oblique direction, keeping the contrast of asufficiently high value of 10 or more in practical use when viewed froman oblique direction.

Specifically, when retardation Rtac [nm] in the thickness direction ofthe TAC films 12 b and 13 b and a parameter α1 [nm] in relation to theretardation Rp are expressed by the following expression (3):α1=35+(Rlc/80−4)²×3.5+(360−Rlc)×Rtac/850  (3),the retardation Rp in the in-plane direction of the positive uniaxialfilm 14 is set to fall within the range of not less than 80% to morethan 120% of the parameter α1.

Further, when retardation Rlc [nm] in the thickness direction of theliquid crystal cell 11 and a parameter β1 [nm] in relation to theretardation Rn are expressed by the following expression (4):β1=Rlc−1.9×Rtac  (4),the retardation Rn in the thickness direction of the negative uniaxialfilm 15 is set to fall within the range of not less than 60% to not morethan 90% of the parameter β1.

Thus, by setting the retardations Rp and Rn to be in the range A1 shownin FIG. 5 with reference to the parameters α1 and β1, it is possible tosurely attain the liquid crystal display device 1 with such an excellentviewing angle property that a viewer hardly perceives the coloring andtone degradation when viewing from an oblique direction, maintaining thecontrast of a sufficiently high value of 10 or more in practical usewhen viewed from an oblique direction.

Further, less coloring and tone degradation found by the viewer occur inthe inner part than in the peripheral part of the range A1. Especially,as in a range A2 shown in FIG. 5, by setting the retardation Rp to fallwithin the range from not less than 90% to not more than 110% of theparameter α1 as well as by setting the retardation Rn to fall within therange from not less than 65% to not more than 85% of the parameter β1,it is possible to realize the liquid crystal display device 1 with moreexcellent viewing angle property.

Note that, when the retardations Rp and Rn are set to fall with therange A2, the improvement in the coloring and tone degradation more thanthat obtained when those are set to fall within the range A1 is notrecognized by the viewer, and the improvement in the coloring and tonedegradation is substantially saturated. Therefore, by setting theretardations Rp and Rn to be in the range A2, it is possible to realizethe liquid crystal display device 1 with such an excellent displayquality level. Further, when the retardations Rp and Rn are respectivelyset to the same values of the parameters α1 and β1, the contrast whenviewed from an oblique direction becomes the maximum. Still further, theretardations Rp and Rn respectively set to the values ranging from 80%to 120% of the parameter α1 and 85% to 90% of the parameter β1 cansuppress the occurrence of the coloring and tone degradation withinallowing limits as well as enhance the contrast, as compared with thecase where the retardations Rp and Rn are set to fall within the rangeA2.

Here, as obvious from the expression (5), the increase or degrease inthe optimal value of the retardation Rp in the in-plane direction of thepositive uniaxial film 14 according to the retardation Rtac in thethickness direction of the TAC films 12 b and 13 b varies depending onthe retardation Rlc in the thickness direction of the liquid crystalcell 11. On the border of the retardation Rlc of 360 [nm] of the liquidcrystal cell 11, dependency of the optimal retardation Rp on theretardation Rtac is reversed.

Therefore, by setting the retardation Rlc in the thickness direction ofthe liquid crystal cell 11 to 360 [nm], it is possible to fix theretardation Rp in the in-plane direction of the positive uniaxial film14 to 35.9 [nm] regardless of the retardation Rtac.

Further, in the case where the retardation Rlc is in the range from 324[nm] to 396 [nm] and the retardation Rp is in the range from 30.7 [nm]to 41.7 [nm], the retardation Rp is within the range from 80% to 120% ofthe parameter α1 under the condition that the retardation Rtac is ageneral value, i.e. approximately 30 [nm] to 80 [nm]. As a result ofthis, by setting the retardation Rn to be 60% to 90% of the parameterβ1, it is possible to surely attain the liquid crystal display device 1with such an excellent viewing angle property that a viewer hardlyperceives the coloring and tone degradation when viewing from an obliquedirection, maintaining the contrast of a sufficiently high value of 10or more in practical use when viewed from an oblique direction.

Therefore, in the case where it is placed importance on the improvementin productivity, it is desirable that the retardation Rlc in thethickness direction of the liquid crystal cell 11 is set to fall withinthe range from 324 [nm] to 396 [nm], and the retardation Rp in thein-plane direction of the positive uniaxial film 14 is set to fallwithin the range from 30.7 [nm] to 41.7 [nm].

According to this, even in the case where the retardation Rtac changesdepending on the variations of the TAC films 12 b and 13 b caused in themanufacturing process, it is possible to realize the liquid crystaldisplay device 1 b with such an excellent viewing angle property asdescribed above, using the positive uniaxial film 14 with the same valueof the retardation Rp in the in-plane direction. As a result of this,even in the case of the variations of the TAC films 12 b and 13 b causedin the manufacturing process, it is possible to fix a type of thepositive uniaxial film 14, thus improving the productivity.

Further, in the case where it is placed importance on both theimprovement in productivity and more excellent viewing angle property,it is desirable that the retardation Rlc is set to fall within the rangefrom 342 [nm] to 378 [nm], and the retardation Rp is set to fall withinthe range from 33.3 [nm] to 38.6 [nm]. In this case, the retardation Rpis within the range from 90% to 110% of the parameter α1 under thecondition that the retardation Rtac is a general value, i.e.,approximately 30 [nm] to 80 [nm]. Therefore, by setting the retardationRn to fall within the range from 65% to 85% of the parameter β1, it ispossible to realize the liquid crystal display device 1 having valueswithin the range A2, i.e., the liquid crystal display device 1 with anextremely excellent viewing angle property. Also in this case, even inthe case where the retardation Rtac varies depending on the variationsof the TAC films 12 b and 13 b caused in the manufacturing process, itis possible to fix a type of the positive uniaxial film 14, thusimproving the productivity.

EXAMPLE 1

In the present example, as the liquid crystal cell 11 prepared wereliquid crystal cells with a refractive index anisotropy Δn of 0.08 eachin the liquid crystal layer 11 c, and respective thicknesses (cellthickness dlc) of 3.0 [μm], 4.0 [μm], and 5.0 [μm], i.e., liquid crystalcells with respective retardations Rlc (=dlc·Δn) in the thicknessdirection of 240 [nm], 320 [nm], and 400 [nm]. Also, as TAC films 12 band 13 b prepared were TAC films with respective retardations Rtac inthe thickness direction of 0 [nm], 30 [nm], 50 [nm], and 80 [nm].Further, for all combinations of the liquid crystal cells 11 with theTAC films 12 b and 13 b calculated were the respective retardations Rpand Rn where the contrast when viewed from an oblique direction becamethe maximum. As a result of this, the experimental result shown in FIG.7 could be obtained.

Note that, a viewing angle in the case where the liquid crystal device 1is actually used is an angle (polar angle) from the normal to the liquidcrystal cell 11 in the range from 0 degree to 60 degrees. Because theincrease of the polar angle degrades the contrast, the contrast wasmeasured from the direction in which the polar angle is 60 degrees, asshown in FIG. 7. Further, the contrast was measured at an azimuth(in-plane direction) of 45 degrees with reference to the absorption axesAA12 and AA13 of the polarizing films 12 a and 13 a because the contrastbecomes the minimum at the azimuth of 45 degrees with reference to theabsorption axes AA12 and AA13.

As result of this, in the arrangement in which the positive uniaxialfilm 14 was layered between the negative uniaxial film 15 and the liquidcrystal cell 11, as shown in FIG. 1, it was confirmed that the liquidcrystal display device 1 with the maximum contrast could be attainedunder the condition that the retardation Rp in the in-plane direction ofthe positive uniaxial film 14 was equal to the foregoing parameter α1and the retardation Rn in the thickness direction of the negativeuniaxial film 15 was equal to the foregoing parameter β1. Further, theforegoing expressions (3) and (4) could be calculated from theexperimental result.

Further, it was confirmed that in the case of the liquid crystal cells11 prepared above, using the above-prepared general TAC films 12 b and13 b (Rtac=30, 50, 80 [nm]), the optimal value of the retardation Rp inthe in-plane direction of the positive uniaxial film 14 was from 35 to49 [nm], and in the case where the thickness of the liquid crystal cell11 was 3.0 [μm] and 4.0 [μm], that is, in the case where the retardationRlc in the thickness direction of the liquid crystal cell 11 was 240[nm] and 320 [nm], the optimal value of the retardation Rp increasedwith the increase in the retardation Rtac. It was also confirmed that inthe case where the thickness of the crystal cell 11 was 5.0 [μm] (theretardation Rlc was 400 [nm]), the optimal value of the retardation Rpdecreased with the increase in the retardation Rtac.

Further, by setting the retardation Rlc in the thickness direction ofthe liquid crystal cell 11 to 360 [nm], it was confirmed that theretardation Rp where the contrast became the maximum when viewed fromthe foregoing oblique direction is approximately constant in spite ofthe change in the retardation Rtac.

In addition, with changes in the retardations Rp and Rn by 5% each at atime, a viewer from the oblique direction estimated the coloring andtone degradation caused in each of the liquid crystal display devices 1.Specifically, the viewer from the oblique direction judged, as thepresence or absence of a coloring phenomenon, whether there occurred thephenomenon that white shifted to yellow or a bluish color, and judged,as the presence or absence of tone degradation, whether there occurredthe phenomenon that tone degradation in bright regions deteriorated theexpressiveness of images.

According to this judgment, it was confirmed that even in the case wherethe retardation Rlc in the thickness direction of the liquid crystalcell 11 and the retardation Rtac of the TAC films 12 b and 13 b had anyof the foregoing values, the contrast from the oblique direction (at thepolar angle of 60 degrees) was above 10, maintaining a sufficientcontrast in practical use, under the condition that the retardation Rpwas a value of not less than 80% nor more than 120% of the parameter α1,and the retardation Rn was a value not less than 60% nor more than 90%of the parameter β1. Further, it was confirmed that when theretardations Rp and Rn were set to the foregoing ranges, the liquidcrystal display device 1 indicated such an excellent viewing angleproperty that the viewer hardly perceived the coloring and tonedegradation when viewing from the oblique direction. Further, in thecase where the retardation Rp was smaller than 80% or greater than 120%of the parameter α1, and in the case where the retardation Rn wassmaller than 60% or greater than 90% of the parameter β1, it was clearlyconfirmed by the viewer from the oblique direction that there occurredthe coloring phenomenon that white shifted to yellow or a bluish color,or the phenomenon that tone degradation in bright regions deterioratedthe expressiveness of images, and it was confirmed as well that thecoloring and tone degradation was not tolerable for the viewer.

In addition, it was confirmed that even in the case where theretardation Rlc in the thickness direction of the liquid crystal cell 11and the retardation Rtac of the TAC films 12 b and 13 b had any of theforegoing values, less coloring and tone degradation was found by theviewer from the oblique direction under the condition that theretardation Rp was a value not less than 90% nor more than 110% of theparameter α1 and the retardation Rn was a value not less than 65% normore than 85% of the parameter β1 than the condition that theretardation Rp was a value ranging from 80% to 90% or from 110% to 120%of the parameter α1 and the retardation Rn was a value ranging from 60%to 65% or from 85% to 90% of the parameter β1.

Under the condition that the retardation Rp was a value not less than90% not more than 110% of the parameter α1 and the retardation Rn was avalue not less than 65% nor more than 85% of the parameter β1, it wasconfirmed that the effect of the improvement in the coloring and tonedegradation was substantially saturated, and a plurality of liquidcrystal display devices 1 with the respective Rp and Rn set to the aboverange could obtain similarly excellent display quality level so that theviewer from the oblique direction could not recognize the differences inthe coloring and tone degradation from the liquid crystal displaydevices 1.

Note that, it was confirmed that the center value of the retardation Rpin the range A2 was a value of 100% of the retardation Rp that maximizedthe contrast from the oblique direction (=the parameter α1) (a valueequal to the parameter α1). Meanwhile, it was also confirmed that thecenter value of the retardation Rn in the range A2 was 75% of theretardation Rn that maximized the contrast from the oblique direction(=the parameter β1), and the coloring phenomenon and the tonedegradation could be improved under the condition that the retardationRn in the thickness direction of the negative uniaxial film 15 was setto be a value smaller than the parameter β1 where the contrast becamethe optimal.

Also, it was confirmed that the condition that the retardation Rp wasset to fall within the range from 80% to 120% of the parameter α1, andthe retardation Rn was set to fall within the range from 85% to 90% ofthe parameter β1 could limit the coloring and tone degradation withinallowable limits as well as enhance the contrast, as compared with thecondition set to fall within the range A2.

Further, by setting the retardation Rlc of the liquid crystal cell 11 tobe in the range from 324 [nm] to 396 [nm] and setting the retardation Rpin the in-plane direction of the positive uniaxial film 14 to be in therange from 30.7 [nm] to 41.7 [nm], it was confirmed that the contrastwhen the liquid crystal display device 1 was viewed from an obliquedirection was above 10 under the condition that the retardation Rtac wasa general value, and that the viewer from the oblique direction hardlyperceives the coloring and tone degradation. Also, it was confirmed thatin terms of coloring and tone degradation, the viewer from an obliquedirection could not recognize the differences from the liquid crystaldisplay devices 1 with the values of the retardations Rp and Rn withinthe range A2 under the condition that the retardation Rlc was in therange from 342 [nm] to 378 [nm], and the retardation Rp was in the rangefrom 33.3 [nm] to 38.6 [nm].

SECOND EMBODIMENT

A liquid crystal display device 1 a according to the present embodimenthas an arrangement similar to that of the liquid crystal display device1 shown in FIG. 1. However, instead of the positive uniaxial film 14 andthe negative uniaxial film 15, a biaxial film (phase difference film) 16is layered between the liquid crystal cell 11 and the polarizing plate12 shown in FIG. 8.

The biaxial film 16 is an optically anisotropic film having the propertyof nx2>ny2>nz2, where nx2 and ny2 are refractive indexes in the in-planedirection of the film, and nz2 is a refractive index in the normaldirection. Let a film thickness be d2, retardation Rxy in the in-planedirection and retardation Rz in the thickness direction can becalculated by the following respective expressions (5) and (6):Rxy=d2·(nx2−ny2)  (5)Rz=d2·{(nx2+ny2)/2−nz2}  (6)Further, the biaxial film 16 is disposed so that its in-planeretardation axis SL16 is orthogonal to the absorption axis AA12 of thepolarizing plate 12 at the same side when viewed from the liquid crystalcell 11.

Also in such a case, in the case where a liquid crystal cell 11 in whichliquid crystal molecules are vertically oriented is viewed from anoblique direction, the biaxial film 16 compensates the phase differencethat the liquid crystal cell 11 brings to transmitting light. Therefore,if the retardation of the biaxial film 16 is properly set, the contrastwhen viewed from an oblique direction can be enhanced.

Further, in the liquid crystal display device 1 a according to thepresent embodiment, the retardation of the biaxial film 16 is set asdescribed below, in order to attain a liquid crystal display devicewhich characterizes excellent color and gradation, maintaining asufficiently high contrast in practical use, as a display quality levelwhen viewed obliquely; more specifically, in order to attain a liquidcrystal display device such that a viewer hardly perceives the coloringand tone degradation when viewing from an oblique direction, keeping thecontrast of a sufficiently high value of 10 or more in practical usewhen viewed from an oblique direction.

Specifically, when retardation Rtac [nm] in the thickness direction ofthe TAC films 12 b and 13 b and a parameter α2 [nm] in relation to theretardation Rxy in the in-plane direction are expressed by the followingexpression (7):α2=85−0.09×Rlc−Rtac/20  (7),the retardation Rxy in the in-plane direction of the biaxial film 16 isset to fall within the range from not less than 80% to not more than120% of the parameter α2.

Further, when retardation Rlc [nm] in the thickness direction of theliquid crystal cell 11 and a parameter β2 [nm] in relation to theretardation Rz are expressed by the following expression (8):β2=1.05×Rlc−1.9×Rtac  (8),the retardation Rz in the thickness direction of the biaxial film 16 isset to fall within the range from not less than 60% to not more than 90%of the parameter β2.

Thus, by setting the retardations Rxy and Rz to fall within the range A1shown in FIG. 9 with reference to the parameters α2 and β2, it ispossible to surely attain the liquid crystal display device 1 a withsuch an excellent viewing angle property that a viewer hardly perceivesthe coloring and tone degradation when viewing from an obliquedirection, maintaining the contrast of a sufficiently high value of 10or more in practical use when viewed from an oblique direction.

Further, less coloring and tone degradation found by the viewer occur inthe inner part than in the peripheral part of the range A1. Especially,as in the range A2 shown in FIG. 9, by setting the retardation Rxy to benot less than 90% nor more than 110% of the parameter α2 as well as bysetting the retardation Rz to fall within the range from not less than65% to not more than 85% of the parameter β2, it is possible to realizethe liquid crystal display device 1 a with more excellent viewing angleproperty.

Note that, when the retardations Rxy and Rz are set to fall within therange A2, the improvement in the coloring and tone degradation more thanthat obtained when those are set to fall within the range A1 is notrecognized by the viewer, and the improvement in the coloring and tonedegradation is substantially saturated. Therefore, by setting theretardations Rxy and Rz to be in the range A2, it is possible to realizethe liquid crystal display device 1 a with such an excellent displayquality level. Further, when the retardations Rxy and Rz arerespectively set to the same values of the parameters α2 and β2, thecontrast when viewed from an oblique direction becomes the maximum.Still further, the retardations Rxy and Rz respectively set to thevalues ranging from 80% to 120% of the parameter α2 and 85% to 90% ofthe parameter β2 can suppress the occurrence of the coloring and tonedegradation within allowable limits as well as enhance the contrast, ascompared with the case where the retardations Rxy and Rz are set to fallwithin the range A2.

As a liquid crystal display device 1 b shown in FIG. 10, biaxial films16 a and 16 b into which the biaxial film 16 shown in FIG. 8 is dividedmay be disposed on the two sides of the liquid crystal cell 11,respectively. Note that, in this case, the biaxial films 16 a and 16 brespectively correspond to first and second phase difference filmsrecited in the claims.

In this case, the biaxial film 16 a is disposed so that its retardationaxis SL16 a in the in-plane direction is orthogonal to the absorptionaxis AA12 of the polarizing plate 12 at the same side when viewed fromthe liquid crystal cell 11. Similarly, the biaxial film 16 b is disposedso that its retardation axis SL16 b is orthogonal to the absorption axisAA13 of the polarizing plate 13 at the same side when viewed from theliquid crystal cell 11. Also in this case, the same effect can beobtained by setting each of the retardations Rxya and Rxyb in thein-plane direction of the respective biaxial films 16 a and 16 b to behalf of the retardation Rxy in the in-plane direction of the biaxialfilm 16 and setting each of the retardations Rza and Rzb in thethickness direction of the respective biaxial films 16 a and 16 b to behalf of the retardation Rz in the thickness direction of the biaxialfilm 16.

Specifically, when retardation Rtac [nm] in the thickness direction ofthe TAC films 12 b and 13 b and a parameter α3 [nm] in relation to theretardations Rxya and Rxyb in the in-plane direction are expressed bythe following expression (9):α3=42.5−0.045×Rlc−Rtac/40  (9),each of the retardations Rxya and Rxyb in the in-plane direction of therespective biaxial films 16 a and 16 b are set to fall within the rangefrom not less than 80% to nor more than 120% of the parameter α3.

Further, when retardation. Rlc [nm] in the thickness direction of theliquid crystal cell 11 and a parameter β3 [nm] in relation to theretardations Rza and Rzb are expressed by the following expression (10):β3=0.525×Rlc−0.95×Rtac  (10),each of the retardations Rza and Rzb in the thickness direction of therespective biaxial films 16 a and 16 b are set to fall within the rangefrom not less than 60% to not more than 90% of the parameter β3.

As in the case of the liquid crystal display device 1 a, this makes itpossible to surely attain the liquid crystal display device 1 b withsuch an excellent viewing angle property that a viewer hardly perceivesthe coloring and tone degradation when viewing from an obliquedirection, maintaining the contrast of a sufficiently high value of 10or more in practical use when viewed from an oblique direction.

Further, as in the case of the liquid crystal display device 1 a, lesscoloring and tone degradation found by the viewer occur in the innerpart than in the peripheral part of the range A1. Especially, as in therange A2 shown in FIG. 9, by setting the retardations Rxya and Rxyb tobe not less than 90% nor more than 110% of the parameter α3 as well asby setting the retardations Rza and Rzb to be not less than 65% nor morethan 85% of the parameter β3, it is possible to realize the liquidcrystal display device 1 b with more excellent viewing angle property.

Note that, as in the case of the liquid crystal display device 1 a, whenthe retardations Rxya and Rxyb and the retardations Rza and Rzb are setto fall within the range A2, the improvement in the coloring and tonedegradation more than that obtained when those are set to fall withinthe range A1 is not recognized by the viewer, and the improvement in thecoloring and tone degradation is substantially saturated. Therefore, bysetting the retardations Rxya and Rxyb and the retardations Rza and Rzbto be in the range A2, it is possible to realize the liquid crystaldisplay device 1 b with such an excellent display quality level.Further, when the retardations Rxya and Rxyb and the retardations Rzaand Rzb are respectively set to the same values of the parameters α3 andβ3, the contrast when viewed from an oblique direction becomes themaximum. Still further, the retardations Rxya and Rxyb and theretardations Rza and Rzb respectively set to the values ranging from 80%to 120% of the parameter α3 and 85% to 90% of the parameter β3 cansuppress the occurrence of the coloring and tone degradation withinallowable limits as well as enhance the contrast, as compared with thecase where the retardations Rxy and Rz are set to fall within the rangeA2.

EXAMPLE 2

In the present example, the same liquid crystal cells 11 and the TACfilms 12 b and 13 b were prepared as in the Example 1. For allcombinations of the liquid crystal cells 11 with the TAC films 12 b and13 b calculated were the respective retardations Rxy and Rz where thecontrast when viewed from the same oblique direction as that in Example1 became the maximum. According to this, the experimental result shownin FIG. 11 could be obtained.

As shown in FIG. 8, in the arrangement in which the biaxial film 16 waslayered between one of the polarizing plates 12 and 13 (the polarizingplate 12 in FIG. 12) and the liquid crystal cell 11, it was confirmedthat the liquid crystal display device 1 a with the maximum contrastcould be attained under the condition that the retardation Rxy in thein-plane direction of the biaxial film 16 was equal to the foregoingparameter α2 and the retardation Rz in the thickness direction of thebiaxial film 16 was equal to the foregoing parameter β2. Further, byapproximating the experimental result with a linear expression, theforegoing expressions (7) and (8) could be calculated.

Further, it was confirmed that in the case of the liquid crystal cells11 prepared above, using the above-prepared general TAC films 12 b and13 b (Rtac=30, 50, 80 [nm]), the optimal value of the retardation Rxy inthe in-plane direction was from 45 to 65 [nm], and although theretardation Rtac was a retardation in the thickness direction, theretardation Rtac was influential to the retardation Rxy in the in-planedirection of the biaxial film 16, and it was impossible to easily handlethe influence of the TAC films 12 b and 13 b.

In addition, with changes in the retardations Rxy and Rz by 5% each at atime, a viewer from the oblique direction estimated the coloring andtone degradation caused in each of the liquid crystal display devices 1.Specifically, the viewer from the oblique direction judged, as thepresence or absence of a coloring phenomenon, whether there occurred thephenomenon that white shifted to yellow or a bluish color, and judged,as the presence or absence of tone degradation, whether there occurredthe phenomenon that tone degradation in bright regions deteriorated theexpressiveness of images.

According to this judgment, it was confirmed that even in the case wherethe retardation Rlc in the thickness direction of the liquid crystalcell 11 and the retardation Rtac of the TAC films 12 b and 13 b had anyof the foregoing values, the contrast from the oblique direction (at thepolar angle of 60 degrees) was above 10, maintaining a sufficientcontrast in practical use, under the condition that the retardation Rxywas a value of not less than 80% nor more than 120% of the parameter α2,and the retardation Rz was a value not less than 60% nor more than 90%of the parameter β2. Further, it was confirmed that when theretardations Rxy and Rz were set to the foregoing ranges, the liquidcrystal display device 1 a indicated such an excellent viewing angleproperty that the viewer hardly perceived the coloring and tonedegradation when viewing from the oblique direction. Further, in thecase where the retardation Rxy was smaller than 80% or greater than 120%of the parameter α2, and in the case where the retardation Rz wassmaller than 60% or greater than 90% of the parameter β2, it was clearlyconfirmed by the viewer from the oblique direction that there occurredthe coloring phenomenon that white shifted to yellow or a bluish color,or the phenomenon that tone degradation in bright regions deterioratedthe expressiveness of images, and it was confirmed as well that thecoloring and tone degradation was not tolerable for the viewer.

In addition, it was confirmed that even in the case where theretardation Rlc in the thickness direction of the liquid crystal cell 11and the retardation Rtac of the TAC films 12 b and 13 b had any of theforegoing values, less coloring and tone degradation was found by theviewer from the oblique direction under the condition that theretardation Rxy was a value not less than 90% nor more than 110% of theparameter α2 and the retardation Rz was a value not less than 65% normore than 85% of the parameter β2 than the condition that theretardation Rxy was a value ranging from 80% to 90% or from 110% to 120%of the parameter α2 and the retardation Rz was a value ranging from 60%to 65% or from 85% to 90% of the parameter β2.

Under the condition that the retardation Rxy was a value not less than90% not more than 110% of the parameter α2 and the retardation Rz was avalue not less than 65% nor more than 85% of the parameter β2, it wasconfirmed that the effect of the improvement in the coloring and tonedegradation was substantially saturated, and a plurality of liquidcrystal display devices 1 a with the respective Rxy and Rz set to fallwithin the above range could obtain similarly excellent display qualitylevel so that the viewer from the oblique direction could not recognizethe differences in the coloring and tone degradation from the liquidcrystal display devices 1 a.

Note that, it was confirmed that the center value of the retardation Rxyin the range A2 was a value of 100% of the retardation Rxy thatmaximized the contrast from the oblique direction (=the parameter α2) (avalue equal to the parameter α2). Meanwhile, it was also confirmed thatthe center value of the retardation Rz in the range A2 was 75% of theretardation Rz that maximized the contrast from the oblique direction(=the parameter β2), and the coloring phenomenon and the tonedegradation could be improved under the condition that the retardationRz in the thickness direction of the biaxial film 16 was set to be avalue smaller than the parameter β2 where the contrast became theoptimal.

Also, it was confirmed that the condition that the retardation Rxy wasset to fall in the range from 80% to 120%, and the retardation Rz wasset to fall in the range from 85% to 90% could suppress the coloring andtone degradation within allowable limits as well as enhance thecontrast, as compared with the condition set to fall within the rangeA2.

Also in the arrangement in which the biaxial film 16 was divided intotwo films, as the liquid crystal display device 1 b shown in FIG. 10, itwas confirmed that even when the retardations Rlc and Rtac were any ofthe foregoing values, each of the retardations Rxya, Rxyb, Rza, and Rzbfor obtaining the maximum contrast at the oblique viewing angle (polarangle of 60 degrees) were half of the values in the liquid crystaldisplay device 1 a shown in FIG. 11, and that with reference to theparameters α3 and β3, instead of α2 and β2, it is possible to obtain thesame effect in the same range as the liquid crystal display device 1 a.Specifically, by setting the retardations Rxya and Rxyb to be in therange from 80% to 120% of the parameter α3 and setting the retardationsRza and Rzb to be in the range from 60% to 90% of the parameter β3, itwas confirmed that it is possible to suppress the coloring and tonedegradation within allowable limits when viewed at the oblique viewingangle (polar angle of 60 degrees). Further, by setting the retardationsRxya and Rxyb to be in the range from 90% to 110% of the parameter α3and setting the retardations Rza and Rzb to be in the range from 65% to85% of the parameter β3, it was confirmed that the improvement in thecoloring and tone degradation is saturated at the oblique viewing angle,and it is possible to obtain the liquid crystal display device 1 b withsimilarly excellent display quality level. Still further, by setting theretardations Rxya and Rxyb to be in the range from 80% to 120% of theparameter α3 and setting the retardations Rza and Rzb to be in the rangefrom 85% to 90% of the parameter β3, it was confirmed that it ispossible to enhance the contrast, suppressing the coloring and tonedegradation within allowable limits when viewed at the oblique viewingangle.

Note that, explained in the foregoing first and second embodiments isthe case where the orientation direction of the liquid crystal moleculesin the pixel is divided into four directions in the liquid crystal cell11 arranged as shown in FIGS. 2 though 4. However, the present inventionis not limited to this. For example, other structures, such asstructures shown in FIGS. 12 and 13, in which the orientation directionis divided into four directions, can also obtain the same effect.

More specifically, a liquid crystal cell using a pixel electrode 21 ashown in FIG. 12 is provided with a quadrangular pyramid-shapedprotrusion 24 formed on the pixel electrode 21 a, instead of theprotrusions 23 a and 23 b shown in FIG. 4. Note that, the protrusion 24,as the protrusions 23 a, can be formed by the application of aphotosensitive resin on the pixel electrode 21 a and the process ofphotolithography.

Also in this arrangement, the liquid crystal molecules near theprotrusion 24 are oriented so as to be vertical to each of the slopes.In addition, when a voltage is applied, the electric filed in the partof the protrusion 24 tilts in the parallel direction to the slope of theprotrusion 24. As a result of these, when a voltage is applied, thein-plane component of the orientation angle in the liquid crystalmolecules is equal to the in-plane component in the normal direction tothe slope that is the nearest to the liquid crystal molecules(directions P1, P2, P3 or P4). Therefore, the pixel region is dividedinto four domains D1 to D4 of mutually different orientation directionswhen the liquid crystal molecules tilt. As a result of this, it ispossible to obtain the same effect as that of the liquid crystal cell 11with the structure shown in FIGS. 2 through 4.

Note that, in the case where a large-size liquid crystal television suchas a 40-inch liquid crystal television, for example, is manufactured,the size of each pixel becomes as large as 1 mm square, and oneprotrusion 24 alone provided each on the pixel electrode 21 a producesweek orientation control force, which may cause an unstable orientation.Thus, as in this case, in the case where orientation control force isinsufficient, it is desirable that a plurality of protrusions 24 areprovided on each of the pixel electrode 21 a.

Further, a multi-domain vertical alignment can be also realized with thearrangement in which an orientation control window 25 where Y-shapedslits are connected symmetrically in the up-and-down direction (thein-plane direction that is parallel to any of the sides of thesubstantially square pixel electrode 21 a) is provided on the counterelectrode 21 b of the counter substrates 11 b, for example, as shown inFIG. 13.

When a voltage is applied, the foregoing arrangement does not produceenough electric field to tilt the liquid crystal molecules in the regionright below the orientation control window 25, of the surface area ofthe counter substrate 11 b, and the liquid crystal molecules arevertically oriented. On the other hand, in the region surrounding theorientation control window 25, of the surface area of the countersubstrate 11 b, as it goes close to the counter substrate 11 b, anelectric field spreads so as to escape from the orientation controlwindow 25. Here, the long axes of the liquid crystal molecules tilt inthe vertical direction to the electric field, and the in-plane componentin the orientation direction of the liquid crystal molecules becomesubstantially vertical to each side of the orientation control window25, as indicated by arrows in FIG. 13. Therefore, also in thisarrangement, the orientation direction of the liquid crystal moleculesin the pixel can be divided into four directions, and it is possible toobtain the same effect as that of the liquid crystal cell 11 with thestructure shown in FIGS. 2 though 4.

Further, explained in the above description is the case where theorientation direction is divided into four directions. The structureusing a radial alignment liquid crystal cell 11, as shown in FIGS. 14and 15, can also obtain the same effect.

More specifically, in the structure shown in FIG. 14, instead of theprotrusion 24 shown in FIG. 12, a substantially hemispherical protrusion26 is provided. Also in this case, the liquid crystal molecules near theprotrusion 26 are oriented so as to be vertical to the surface of theprotrusion 26. In addition, when a voltage is applied, an electric fieldin the part of the protrusion 26 tilts in the parallel direction to thesurface of the protrusion 26. From these results, the liquid crystalmolecules, in tilting when a voltage is applied, tend to tilt in aradial pattern about the protrusion 26 in the in-plane direction, andthe liquid crystal molecules in the liquid crystal cell 11 can tilt andorient in a radial pattern. Note that, the protrusion 26 can be formedin the same process as that of the protrusion 24. Further, as theprotrusion 24, in the case where orientation control force isinsufficient, it is desirable that a plurality of protrusions 26 areprovided on each of the pixel electrodes 21 a.

In the structure shown in FIG. 15, instead of the protrusion 24 shown inFIG. 12, a circular slit 27 is provided to the pixel electrode 21 a.When a voltage is applied, this arrangement does not produce enoughelectric field to tilt the liquid crystal molecules in the region righton the slit 27, of the surface of the pixel electrode 21 a. Therefore,in this region, even when a voltage is applied, the liquid crystalmolecules are oriented vertically. On the other hand, in the region nearthe slit 27, of the surface of the pixel electrode 21 a, as it getsclose to the slit 27 in the thickness direction, the electric fieldspreads obliquely so as to escape from the slit 27. Here, the long axesof the liquid crystal molecules tilt in the vertical direction.According to the continuity of liquid crystal, the liquid crystalmolecules away from the slit 27 are also oriented in the same direction.Thus, when a voltage is applied to the pixel electrode 21 a, the liquidcrystal molecules can be oriented so that the in-plane components of theorientation direction spread in a radial pattern about the slit 27 asindicated by arrows in FIG. 15, that is, the liquid crystal moleculescan be oriented symmetrically to the center of the slit 27. Here, sincethe tilt of the electric field varies depending on applied voltage, asubstrate's normal direction component (tilt angle) in the orientationdirection of the liquid crystal molecules can be controlled by appliedvoltage. Note that, since the increase in applied voltage increases atilt angle to the substrate's normal direction, the liquid crystalmolecules are oriented substantially in parallel with a display screenas well as in a radial pattern in a plane. Further, as the protrusion26, in the case where orientation control force is insufficient, it isdesirable that a plurality of slits 27 are provided on each of the pixelelectrodes 21 a.

Incidentally, explained in the above description is the case where theorientation direction of the liquid crystal molecules in the pixel isdivided. However, a liquid crystal cell without orientation division (amono-domain liquid crystal cell) can also obtain the same effect.

In this case, the pixel electrode 21 a and the counter electrode 21 b,which are not provided with the protrusions 23 a and others, are formedevenly. In the mono-domain vertical alignment liquid crystal cell,unlike a multi-domain vertical alignment or radial tilt alignment liquidcrystal cell, a rubbing process is included in the manufacturingprocess, and rubbing directions of the liquid crystal molecules in theliquid crystal layer 11 c are set so as not to be parallel between thesubstrates 11 a and 11 b. The liquid crystal cell 11 and the polarizingplate 12 and 13 are disposed so that the rubbing direction forms anangle of 45 degrees with the absorption axes AA12 and AA13 of therespective polarizing plates 12 and 13. Also in this case, when novoltage is applied, the liquid crystal molecules in the pixel areoriented in the substrate's normal direction (in the verticaldirection), as in the case of FIG. 2. Therefore, it is possible toobtain the same effect by using the same polarizing plates 12 and 13 andphase difference plates (14 to 16, 16 a and 16 b) as those in the aboveembodiments.

Incidentally, according to the liquid crystal display devices 1 and 1 ashown in FIGS. 1 and 8, respectively, the optical properties of membersdisposed from the liquid crystal cell 11 to the polarizing plate 12 onone side are not equal to those of members disposed from the liquidcrystal cell 11 to the polarizing plate 13 on the other side, so thatthere is the possibility that the contrast when the liquid crystal cell11 is viewed from an azimuth on the left side may be different from thatviewed from an azimuth on the right side, and the contrast when theliquid crystal cell 11 is viewed from an azimuth on the upper side maydifferent from that viewed from an azimuth on the bottom side.Therefore, in the case where these liquid crystal display devices 1 and1 a requires the viewing angle property balanced in the left, right,upper, and bottom directions, it is desirable to use a liquid crystalcell in which the orientation direction of the liquid crystal moleculesin each pixel is divided into four or more directions, such as fourdivision alignment liquid crystal cells and radial alignment liquidcrystal cells.

Further, as an example explained in the above description is the casewhere the liquid crystal cell 11 has the liquid crystal layer 11 c witha negative dielectric anisotropy; however, the present invention is notlimited to this. The liquid crystal cell 11 having the liquid crystallayer 11 c with a positive dielectric anisotropy can also obtain thesame effect, provided that the liquid crystal cell is the one in whichthe liquid crystal molecules are oriented vertically to the substrate ofthe liquid crystal cell 11 in black displaying, as the structure in FIG.2.

In this case, an electric field is applied to the liquid crystal layer11 c in the parallel direction to substrates by using an electrode thatproduces an electric field in the parallel direction to substrates, asan electrode of a comblike structure which is used in the IPS (In-PlaneSwitching) mode. Also in this case, when no voltage is applied (whenthere is no electric field), the liquid crystal molecules in the pixelare oriented in the vertical direction to the substrates, as thestructure in FIG. 2. Therefore, it is possible to obtain the same effectby using the same polarizing plates 12 and 13 and the phase differenceplates (14 to 16, 16 a and 16 b) as those in the above embodiments.

As described above, a liquid crystal display device of the presentinvention has an arrangement in which when a parameter α [nm] inrelation to Rp is: α=35+(Rlc/80−4)²×3.5+(360−Rlc)×Rtac/850; and aparameter β [nm] in relation to Rn is: β=Rlc−1.9×Rtac, where Rp [nm] isa retardation in an in-plane direction of the first phase differencefilm, provided between a first polarizing plate and the liquid crystalcell, having a positive uniaxial anisotropy, Rn [nm] is a retardation ina thickness direction of the second phase difference film, providedbetween the first polarizing plate and the first phase difference film,having a negative uniaxial anisotropy, Rtac [nm] is a retardation in athickness direction of the base films of the polarizing plate, and Rlc[nm] is a retardation in a thickness direction of the liquid crystal,the retardation Rp is set to fall within the range from not less than80% to not more than 120% of the parameter α, and the retardation Rn isset to fall within the range from not less than 60% to not more than 90%of the parameter β.

According to this arrangement, the retardations of the first and secondphase difference films are set to fall within the foregoing ranges,respectively. Unlike the case where the retardation in the thicknessdirection of the base films is treated equally to the retardations inthe thickness direction of the first and second phase difference films,this brings about the effect that it is possible to surely obtain aliquid crystal display device which can suppress the coloring and tonedegradation within allowable limits in practical use, while maintaininga contrast at a sufficiently high value in practical use when viewedfrom an oblique direction.

As described above, in addition to the above arrangement, a liquidcrystal display device according to the present invention has anarrangement in which the retardation Rlc in the thickness direction ofthe liquid crystal is set to fall within the range from 324 [nm] to 396[nm], and the retardation Rp in the in-plane direction of the firstphase difference film is set to fall within the range from 30.7 [nm] to41.7 [nm].

Therefore, even in the case where the variations of the base filmscaused in the manufacturing process vary the retardation in thethickness direction of the base films, the retardation Rp can be set tofall within the range from 80% to 120% of the parameter α by setting theretardations Rlc and Rp to be in the foregoing ranges. As a result ofthis, even in the case where the retardation in the thickness directionof the base films varies, it is possible to use the same first phasedifference film. This brings about the effect of improving theproductivity.

As described above, in addition to the above arrangement, a liquidcrystal display device according to the present invention has anarrangement in which the retardation Rp is set to fall within the rangefrom not less than 90% to not more than 110% of the parameter α, and theretardation Rn is set to fall within the range from nor less than 65% tonot more than 85% of the parameter β. This brings about the effect thatit is possible to obtain a liquid crystal display device that canfurther suppress the occurrence of coloring and tone degradation whenviewed from an oblique direction.

As described above, in addition to the above arrangement, a liquidcrystal display device according to the present invention has anarrangement in which the retardation Rlc in the thickness direction ofthe liquid crystal is set to fall within the range from 342 [nm] to 378[nm], and the retardation Rp in the in-plane direction of the firstphase difference film is set to fall within the range from 33.3 [nm] to38.6 [nm].

Therefore, even in the case where the retardation Rtac changes dependingon the variations of the base films caused in the manufacturing processvaries the retardation in the thickness direction of the base films, theretardation Rp can be set to fall within the range from 90% to 110% ofthe parameter α by setting the retardations Rlc and Rp to be in theforegoing ranges. As a result of this, even in the case where theretardation in the thickness direction of the base films varies, it ispossible to use the same first phase difference film. This brings aboutthe effect of improving the productivity.

As described above, a liquid crystal display device of the presentinvention has an arrangement in which when a parameter α [nm] inrelation to Rxy is: α=85−0.09×Rlc−Rtac/20; and a parameter β [nm] inrelation to Rz is: β=1.05×Rlc−1.9×Rtac, where Rxy [nm] is a retardationof a phase difference film, provided between the first polarizing plateand the liquid crystal cell, having a biaxial anisotropy, Rz [nm] is aretardation in the thickness direction of the phase difference film,Rtac [nm] is a retardation in the thickness direction of the base films,and Rlc [nm] is the retardation in the thickness direction of the liquidcrystal, the retardation Rxy is set to fall within the range from notless than 80% to not more than 120% of the parameter α, and theretardation Rz is set to fall within the range from not less than 60% tonot more than 90% of the parameter β.

As described above, a liquid crystal display device of the presentinvention has an arrangement in which when a parameter α [nm] inrelation to Rxy is: α=42.5−0.045×Rlc−Rtac/40; and a parameter β [nm] inrelation to Rz is: β=0.525×Rlc−0.95×Rtac, where Rxy [nm] is aretardation in an in-plane direction of each of the first and secondphase difference films, provided on two sides of the liquid crystalcell, respectively, having a biaxial anisotropy, Rz [nm] is aretardation in the thickness direction of each of the first and secondphase difference films, the retardation Rxy of the first and secondphase difference films is set to fall within the range from not lessthan 80% to not more than 120% of the parameter α, and the retardationRz of the first and second phase difference films is set to fall withinthe range from not less than 60% to not more than 90% of the parameterβ.

In the above-arranged liquid crystal display devices, the retardationsRxy and Ry are set to fall within the foregoing ranges, respectively.Unlike the case where the retardation in the thickness direction of thebase films is treated equally to the retardation of the phase differencefilm or the retardations in the thickness direction of the first andsecond phase difference films, this brings about the effect that it ispossible to surely obtain a liquid crystal display device which cansuppress the coloring and tone degradation within allowable limits inpractical use, while maintaining a contrast at a sufficiently high valuein practical use when viewed from an oblique direction.

As described above, in addition to the above arrangement, a liquidcrystal display device according to the present invention has anarrangement in which the retardation Rxy is set to fall within the rangefrom not less than 90% to not more than 110% of the parameter α, and theretardation Rz is set to fall within the range from not less than 65% tonot more than 85% of the parameter β. This brings about the effect thatit is possible to obtain a liquid crystal display device that canfurther suppress the occurrence of coloring and tone degradation whenviewed from an oblique direction.

As described above, in addition to the above arrangement, a liquidcrystal display device according to the present invention has anarrangement in which regardless of whether or not the above phasedifference films have a biaxial anisotropy, the liquid crystal has anegative dielectric anisotropy.

According to this arrangement, liquid crystal molecules oriented in thenormal direction to the substrate can be tilted depending on electricintensity by applying an electric field substantially in the verticaldirection to the substrate. This can simplify the structure ofelectrodes, as compared with the case of using a liquid crystal having apositive dielectric anisotropy.

A liquid crystal display device according to the present invention,includes:

a liquid crystal cell having a pair of substrates and liquid crystalinterposed therebetween, wherein liquid crystal molecules of said liquidcrystal are oriented substantially vertically to respective surfaces ofsaid pair of substrates;

a pair of polarizing plates disposed so as to sandwich said liquidcrystal cell therebetween, respective absorption axes of said pair ofpolarizing plates being orthogonal to each other;

a first phase difference film, disposed between one of said pair ofpolarizing plates and said liquid crystal cell, said first phasedifference film having a positive uniaxial anisotropy; and

a second phase difference film, disposed between said one of said pairof polarizing plates and said first phase difference film, said secondphase difference film having a negative uniaxial anisotropy,

wherein each of said pair of polarizing plates has a base film with anoptical axis substantially vertical to said pair of substrates, saidbase film having a negative uniaxial anisotropy, said first phasedifference film has a retardation axis crossing at right angle theabsorption axis of said one of said pair of polarizing plates on thesame side when seen from said liquid crystal, and said second phasedifference film has an optical axis substantially vertical to said pairof substrates, taking the following means.

More specifically, when a parameter α [nm] in relation to Rp is:α=35+(Rlc/80−4)²×3.5+(360−Rlc)×Rtac/850; and

a parameter β [nm] in relation to Rn is:β=Rlc−1.9×Rtac,

where Rp [nm] is a retardation in an in-plane direction of the firstphase difference film, Rn [nm] is a retardation in a thickness directionof the second phase difference film, Rtac [nm] is a retardation in athickness direction of the base films, and Rlc [nm] is a retardation ina thickness direction of the liquid crystal, the retardation Rp is setto fall within a range from not less than 80% to not more than 120% ofthe parameter α, and the retardation Rn is set to fall within a rangefrom not less than 60% to not more than 90% of the parameter β.

In the above-arranged liquid crystal display device, liquid crystalmolecules oriented substantially vertically to the substrates, althoughnot bringing a phase difference to light incident from the normaldirection to the substrate, bring a phase difference depending on apolar angle (tilt angle to the normal direction) to obliquely incidentlight. Therefore, the liquid crystal display device cannot completelyabsorb the light supposed to be absorbed by the polarizing plate on theside from where the light emits, without the first and second phasedifference films. This results in the occurrence of light leakage, thusdegrading a contrast and causing the coloring and tone degradation.

In order to solve the problem, since the above arrangement is providedwith first and second phase difference films, the phase difference thatthe liquid crystal has brought depending on the polar angle can becompensated by the first and second phase difference films. As a resultof this, it is possible to prevent light leakage when viewed from anoblique direction, enhancing the contrast and preventing the occurrenceof coloring and tone degradation.

Incidentally, when the respective retardations of the first and secondphase difference films are determined, it cannot be always said thatjust subtracting the retardation in the thickness direction of the basefilms from each of the respective retardations in the thicknessdirection of the first and second phase difference films, which is anoptimum retardation when base films are absent, is sufficient, becausethe coloring and tone degradation caused when viewed from an obliquedirection are required to be suppressed much further.

The inventors of the present application, as a result of extensiveresearch to further suppress the coloring and tone degradation, whilemaintaining a contrast at a sufficiently high value in practical usewhen a vertical alignment mode liquid crystal display device is viewedfrom an oblique direction, have found that the retardation in thethickness direction of the base films does not always function asequally as each of the retardation in the thickness direction of thefirst phase difference film and the retardation in the thicknessdirection of the second phase difference film. Specifically, theinventors have found to complete the present invention that: when theretardation in the in-plane direction of the first phase difference filmwith a positive uniaxial anisotropy is set so that the contrast becomesthe maximum, the dependency of the retardation Rp on the retardation inthe thickness direction of the base films, reverses depending on whetherthe retardation of the liquid crystal is over 360 [nm], and it ispossible to effectively suppress the coloring and tone degradation bysetting the retardations to fall in a predetermined range with referenceto such retardations that the contrast becomes the maximum.

In the liquid crystal display device of the present invention, theretardations Rp and Rn are set according to the retardation Rtac in thethickness direction of the base films and the retardation Rlc in thethickness direction of the liquid crystal, and the retardations Rp andRn are set to fall in the range where the coloring and tone degradationcan be tolerated, while maintaining a contrast at a sufficiently highvalue in practical use when viewed from an oblique direction. With thisarrangement, unlike the arrangement in which the retardation in thethickness direction of the base films is treated equally to theretardation in the thickness direction of the first phase differencefilm and the retardation in the thickness direction of the second phasedifference film, it is possible to surely obtain a liquid crystaldisplay device which can maintain a contrast at a sufficiently highvalue in practical use when viewed from the oblique direction andsuppress the coloring and tone degradation within allowable limits.

In the case where the improvement in productivity is especiallyrequired, in addition to the above arrangement, it is desirable that theretardation Rlc in the thickness direction of the liquid crystal is setto fall within the range from 324 [nm] to 396 [nm], and the retardationRp in the in-plane direction of the first phase difference film is setto fall within the range from 30.7 [nm] to 41.7 [nm].

If the retardation Rlc is set to fall in the foregoing range, thedependency of the retardation Rp on the retardation in the thicknessdirection of the base films is small. Therefore, even in the case wherethe variations of the base films caused in the manufacturing processvaries the retardation in the thickness direction of the base films, theretardation Rp can be set to fall within the range from 80% to 120% ofthe parameter α by setting the retardations Rlc and Rp to fall withinthe foregoing ranges. As a result of this, even in the case where theretardation in the thickness direction of the base films varies, it ispossible to use the same first phase difference film, thus improving theproductivity.

Further, in the case where the suppression of coloring and tonedegradation is especially required, in addition to the abovearrangement, it is desirable that the retardation Rp is set to fallwithin the range from not less than 90% to not more than 110% of theparameter α, and the retardation Rn is set to fall within the range fromnot less than 65% to not more than 85% of the parameter β. With thisarrangement, it is possible to obtain a liquid crystal display devicewhich can further suppress the coloring and tone degradation when viewedfrom an oblique direction.

Still further, in the case where both the suppression of coloring andtone degradation and the improvement in productivity are especiallyrequired, it is desirable that the retardation Rlc in the thicknessdirection of the liquid crystal is set to fall within the range from 342[nm] to 378 [nm], and the retardation Rp in the in-plane direction ofthe first phase difference film is set to fall within the range from33.3 [nm] to 38.6 [nm].

If the retardations Rlc and Rp are set to fall in the foregoing ranges,the retardation Rp can be set to fall in the range from 90% to 110% ofthe parameter α even in the case where the variations of the base filmscaused in the manufacturing process varies the retardation in thethickness direction of the base films. As a result of this, even in thecase where the retardation in the thickness direction of the base filmsvaries, it is possible to use the same first phase difference film, thusimproving the productivity.

Meanwhile, a liquid crystal display device according to the presentinvention, includes:

a liquid crystal cell having a pair of substrates and liquid crystalinterposed therebetween, wherein liquid crystal molecules of said liquidcrystal are oriented substantially vertically to respective surfaces ofsaid pair of substrates;

a pair of polarizing plates disposed so as to sandwich said liquidcrystal cell therebetween, respective absorption axes of said pair ofpolarizing plates being orthogonal to each other; and

a phase difference film, disposed between one of said pair of polarizingplates and said liquid crystal cell, said phase difference film having abiaxial anisotropy,

wherein each of said pair of polarizing plates has a base film with anoptical axis substantially vertical to said pair of substrates, saidbase film having a negative uniaxial anisotropy, and said phasedifference film has an in-plane retardation axis crossing at right anglethe absorption axis of said one of said pair of polarizing plates on thesame side when seen from said liquid crystal, taking the followingmeans.

More specifically, when a parameter α [nm] rel to Rxy is:α=85−0.09×Rlc−Rtac/20; and

a parameter β [nm] in relation to Rz is:β=1.05×Rlc−1.9×Rtac,

where Rxy [nm] is a retardation in an in-plane direction of the phasedifference film, Rz [nm] is a retardation in a thickness direction ofthe phase difference film, Rtac [nm] is a retardation in a thicknessdirection of the base films, and Rlc [nm] is a retardation in athickness direction of the liquid crystal, the retardation Rxy is set tofall within a range from not less than 80% to not more than 120% of theparameter α, and the retardation Rz is set to fall within a range fromnot less than 60% to not more than 90% of the parameter β.

Further, a liquid crystal display device according to the presentinvention, includes:

a liquid crystal cell having a pair of substrates and liquid crystalinterposed therebetween, wherein liquid crystal molecules of said liquidcrystal are oriented substantially vertically to respective surfaces ofsaid pair of substrates;

a pair of polarizing plates disposed so as to sandwich said liquidcrystal cell therebetween, respective absorption axes of said pair ofpolarizing plates being orthogonal to each other;

a first phase difference film, disposed between one of said pair ofpolarizing plates and said liquid crystal cell, said first phasedifference film having a biaxial anisotropy; and

a second phase difference film, disposed between the other of said pairof polarizing plates and said liquid crystal cell, said second phasedifference film having a biaxial anisotropy,

wherein each of said pair of polarizing plates has a base film with anoptical axis substantially vertical to said pair of substrates, saidbase film having a negative uniaxial anisotropy, and each of said firstand second phase difference films has an in-plane retardation axiscrossing at right angle the absorption axis of said one of said pair ofpolarizing plates on the same side when seen from said liquid crystal,taking the following means.

More specifically, when a parameter α [nm] in relation to Rxy is:α=42.5−0.045×Rlc−Rtac/40; and

a parameter β [nm] in relation to Rz is:β=0.525×Rlc−0.95×Rtac,

where Rxy [nm] is a retardation in an in-plane direction of each of thefirst and second phase difference films, Rz [nm] is a retardation in athickness direction of each of the first and second phase differencefilms, Rtac [nm] is a retardation in a thickness direction of the basefilms, and Rlc [nm] is a retardation in a thickness direction of theliquid crystal, the retardation Rxy of the first and second phasedifference films is set to fall within a range from not less than 80% tonot more than 120% of the parameter α, and the retardation Rz of thefirst and second phase difference films is set to fall within a rangefrom not less than 60% to not more than 90% of the parameter β.

In the above-arranged liquid crystal display devices, the phasedifference that the liquid crystal has brought to the light in anoblique direction, in the state where liquid crystal molecules areoriented substantially vertically to the substrates, is compensated bythe phase difference film or the first and second phase differencefilms. This prevents the light leakage when viewed from an obliquedirection, thus enhancing the contrast.

However, even in the foregoing arrangement, when the retardation of thephase difference film or the respective retardations of the first andsecond phase difference films are determined, it cannot be always saidthat just subtracting the retardation in the thickness direction of thebase films from each of the respective retardations in the thicknessdirection of the phase difference films, which is an optimum retardationwhen base films are absent, is sufficient, because further suppressionof the coloring and tone degradation when viewed from an obliquedirection is required.

The inventors of the present application, as a result of extensiveresearch to further enhance the contrast when a vertical alignment modeliquid crystal display device is viewed from an oblique direction, havefound that the retardation in the thickness direction of the base filmsdoes not always function as equally as the retardation in the thicknessdirection of the phase difference film or each of the respectiveretardations in the thickness direction of the first and second phasedifference films, as in the case of the foregoing liquid crystal displaydevice. Specifically, the inventors have found to complete the presentinvention that the retardation Rxy in the in-plane direction of thephase difference film with a biaxial anisotropy and the retardation Rtacin the thickness direction of the base films are different in thedirection of the retardation from each other; however, the influence ofthe retardation Rtac should be also added in order to properly set theretardation Rxy, and that it is possible to effectively suppress thecoloring and tone degradation by setting the retardations to fall in apredetermined range with reference to such retardations that thecontrast becomes the maximum.

In the liquid crystal display device of the present invention, theretardation Rxy in the in-plane direction of the phase difference filmor each of the first and second phase difference films and theretardation Rz in the thickness direction of the phase difference filmor each of the first and second phase difference films are set accordingto the retardation Rlc in the thickness direction of the liquid crystaland the retardation Rtac in the thickness direction of the base films;and the retardations Rxy and Rz are set to fall in the range where thecoloring and tone degradation can be tolerated, while maintaining acontrast at a sufficiently high value in practical use when viewed froman oblique direction. With this arrangement, unlike the case where theretardation in the thickness direction of the base films is treatedequally to the retardation in the thickness direction of the phasedifference film and each of the respective retardations in the thicknessdirection of the first and second phase difference films, it is possibleto surely obtain a liquid crystal display device which can maintain acontrast at a sufficiently high value in practical use when viewed fromthe oblique direction and suppress the coloring and tone degradationwithin allowable limits.

Also, in the case where the suppression of coloring and tone degradationis especially required, in addition to the above arrangement, it isdesirable that the retardation Rxy is set to fall within the range fromnot less than 90% to not more than 110% of the parameter α, and theretardation Rz is set to fall within the range from not less than 65% tonot more than 85% of the parameter β. With this arrangement, it ispossible to obtain a liquid crystal display device which can furthersuppress the coloring and tone degradation when viewed from an obliquedirection.

Further, in the liquid crystal display device according to the presentinvention, it is desirable that the liquid crystal has a negativedielectric anisotropy, regardless of whether or not the phase differencefilms have biaxial anisotropy.

According to this arrangement, liquid crystal molecules oriented in thenormal direction to the substrate can be tilted depending on electricintensity by applying an electric field substantially in the verticaldirection to the substrate. This can simplify the structure ofelectrodes, as compared with the case of using a liquid crystal having apositive dielectric anisotropy.

The invention being thus described, it will be obvious that the same waymay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

INDUSTRIAL APPLICABILITY

As described above, according to the liquid crystal display device ofthe present invention, the retardations Rp and Rn are set according tothe retardation Rtac in the thickness direction of the base films andthe retardation Rlc in the thickness direction of the liquid crystal aswell as set in the range where the coloring and tone degradation can betolerated, while maintaining a contrast at a sufficiently high value inpractical use when viewed from an oblique direction. With thisarrangement, unlike the case where the retardation in the thicknessdirection of the base films is treated equally to each of the respectiveretardations in the thickness direction of the first and second phasedifference films, it is possible to surely obtain a liquid crystaldisplay device which can suppress the coloring and tone degradationwithin allowable limits in practical use, while maintaining a contrastat a sufficiently high value in practical use when viewed from anoblique direction.

1. A liquid crystal display device, comprising: a liquid crystal cellhaving a pair of substrates and liquid crystal interposed therebetween,wherein liquid crystal molecules of said liquid crystal are orientedsubstantially vertically to respective surfaces of said pair ofsubstrates; a pair of polarizing plates disposed so as to sandwich saidliquid crystal cell therebetween, respective absorption axes of saidpair of polarizing plates being orthogonal to each other; a first phasedifference film, disposed between one of said pair of polarizing platesand said liquid crystal cell, said first phase difference film having apositive uniaxial anisotropy; and a second phase difference film,disposed between said one of said pair of polarizing plates and saidfirst phase difference film, said second phase difference film having anegative uniaxial anisotropy, wherein each of said pair of polarizingplates has a base film with an optical axis substantially vertical tosaid pair of substrates, said base film having a negative uniaxialanisotropy, said first phase difference film has a retardation axiscrossing at right angle the absorption axis of said one of said pair ofpolarizing plates on the same side when seen from said liquid crystal,and said second phase difference film has an optical axis substantiallyvertical to said pair of substrates; and when a parameter α [nm] inrelation to Rp is:α=35+(Rlc/80−4)²×3.5+(360−Rlc)×Rtac/850, and a parameter β [nm] inrelation to Rn is:β=Rlc−1.9×Rtac, where Rp [nm] is a retardation in an in-plane directionof said first phase difference film, Rn [nm] is a retardation in athickness direction of said second phase difference film, Rtac [nm] is aretardation in a thickness direction of said base film, and Rlc [nm] isa retardation in a thickness direction of said liquid crystal, saidretardation Rp is set to fall within a range from not less than 80% tonot more than 120% of the parameter α, and said retardation Rn is set tofall within a range from not less than 85% to not more than 90% of theparameter β.
 2. The liquid crystal display device according to claim 1,wherein said liquid crystal has a negative dielectric anisotropy.
 3. Aliquid crystal display device, comprising: a liquid crystal cell havinga pair of substrates and liquid crystal interposed therebetween, whereinliquid crystal molecules of said liquid crystal are orientedsubstantially vertically to respective surfaces of said pair ofsubstrates; a pair of polarizing plates disposed so as to sandwich saidliquid crystal cell therebetween, respective absorption axes of saidpair of polarizing plates being orthogonal to each other; and a phasedifference film, disposed between one of said pair of polarizing platesand said liquid crystal cell, said phase difference film having abiaxial anisotropy, wherein each of said pair of polarizing plates has abase film with an optical axis substantially vertical to said pair ofsubstrates, said base film having a negative uniaxial anisotropy, andsaid phase difference film has an in-plane retardation axis crossing atright angle the absorption axis of said one of said pair of polarizingplates on the same side when seen from said liquid crystal, when aparameter α [nm] in relation to Rxy is:α=85−0.09×Rlc−Rtac/20, and a parameter β [nm] in relation to Rz is:β=1.05×Rlc−1.9×Rtac, where Rxy [nm] is a retardation in an in-planedirection of the phase difference film, Rz [nm] is a retardation in athickness direction of the phase difference film, Rtac [nm] is aretardation in a thickness direction of the base films, and Rlc [nm] isa retardation in a thickness direction of the liquid crystal, saidretardation Rxy is set to fall within a range from not less than 80% tonot more than 120% of the parameter α, and said retardation Rz is set tofall within a range from not less than 60% to not more than 85% of theparameter β.
 4. The liquid crystal display device according to claim 3,wherein said liquid crystal has a negative dielectric anisotropy.
 5. Aliquid crystal display device, comprising: a liquid crystal cell havinga pair of substrates and liquid crystal interposed therebetween, whereinliquid crystal molecules of said liquid crystal are orientedsubstantially vertically to respective surfaces of said pair ofsubstrates; a pair of polarizing plates disposed so as to sandwich saidliquid crystal cell therebetween, respective absorption axes of saidpair of polarizing plates being orthogonal to each other; a first phasedifference film, disposed between one of said pair of polarizing platesand said liquid crystal cell, said first phase difference film having abiaxial anisotropy; and a second phase difference film, disposed betweenthe other of said pair of polarizing plates and said liquid crystalcell, said second phase difference film having a biaxial anisotropy,wherein each of said pair of polarizing plates has a base film with anoptical axis substantially vertical to said pair of substrates, saidbase film having a negative uniaxial anisotropy, and each of said firstand second phase difference films has an in-plane retardation axiscrossing at right angle the absorption axis of said one of said pair ofpolarizing plates on the same side when seen from said liquid crystal;and when a parameter α [nm] in relation to Rxy is:α=42.5−0.045×Rlc−Rtac/40, and a parameter β [nm] in relation to Rz is:β=0.525×Rlc−0.95×Rtac, where Rxy [nm] is a retardation in an in-planedirection of each of said first and second phase difference films, Rz[nm] is a retardation in a thickness direction of each of said first andsecond phase difference films, Rtac [nm] is a retardation in a thicknessdirection of the base films, and Rlc [nm] is a retardation in athickness direction of the liquid crystal, said retardation Rxy of saidfirst and second phase difference films is set to fall within a rangefrom not less than 80% to not more than 120% of the parameter α, andsaid retardation Rz of said first and second phase difference films isset to fall within a range from not less than 85% to not more than 90%of the parameter β.
 6. The liquid crystal display device according toclaim 5, wherein said liquid crystal has a negative dielectricanisotropy.